16.8 Aspergillosis disease of coral

16.8 Aspergillosis disease of coral

Decline of coral reef ecosystems has been a concern since the 1970s. Overall, the loss of coral reefs is thought to be a combined result of global warming, ozone depletion, overfishing, eutrophication, poor land-use practices and other expressions of human activities. There are, however, reports that emphasise the role of coral disease in reef degradation. Many aspects of coral disease are poorly understood, but disease syndromes do seem to be increasing, and may parallel a general decline in the marine environment. However, much of the data is purely descriptive and though viruses, bacteria and fungi have all been implicated as disease organisms most reports lack proof that the alleged pathogen is indeed the cause of the disease disease (Harvell et al., 1999; Work et al., 2008; Woodley et al., 2015).

One disease for which the identity of the pathogen has been proved to a satisfactory level is aspergillosis of sea fans. Aspergillosis is a pathogen of the Caribbean sea fan (Gorgonia spp.) as well as other gorgonian corals. The impact of disease can range from severe, leading to localised mass mortalities, to mild, leading to partial tissue loss and eventual recovery. Gorgonia ventalina sea fans suffered mass mortality on reefs in the Caribbean and the Florida Keys in 1995 and 1996. During the 1995 outbreak, two species of Caribbean sea fans were identified as hosts: Gorgonia ventalina and G. flabellum. The pathogen was identified as Aspergillus sydowii, a common terrestrial soil fungus. Symptoms of the disease included expanding areas of the sea fan in which polyp tissues were destroyed, exposing the axial skeleton beneath. In some cases, the lesions were so severe that holes appeared in the skeleton. The disease was common over a wide geographical area and though partial destruction of individual Gorgonia colonies was most usual, the entire fan was sometimes destroyed. It was demonstrated that tissue-degrading lesions all contained fungal hyphae at their edges and careful analysis demonstrated that sea fan aspergillosis is infectious:

  • the same filamentous Aspergillus species was isolated from different geographical disease areas,
  • the organism could be grown in pure culture in vitro,
  • mycelium taken from culture and inoculated onto healthy sea fans caused typical disease symptoms,
  • the same fungus could be recovered from these experimental disease instances and recultured in vitro (Smith et al., 1996).

The course of action followed here was established by Robert Koch in the 1870s to identify pathogenic microorganisms (they are called Koch’s postulates). Such procedures are necessary to demonstrate unambiguously that a presumed disease pathogen is the true cause of a disease (see discussions in Work et al. (2008). This initial study identified the fungus as Aspergillus fumigatus, but it was later shown to be A. sydowii (Geiser et al., 1998). The disease persists in the western Atlantic and was probably the cause of mass mortality in sea fans that occurred throughout the Caribbean during the 1980s.

Aspergillus sydowii is a common saprotroph that is found in both terrestrial and marine environments. The species is widespread, having been isolated from terrestrial environments as varied as Arctic soils in Alaska and soils in tropical regions, as well as from subtropical marine waters in both coastal regions and oceanic zones as deep as 4,450 m. Although A. sydowii is common and cosmopolitan, it had not previously been recognised as causing disease in plants or animals. However, nonmarine strains of A. sydowii did not cause disease in sea fans so it was concluded that isolates taken from diseased corals have acquired pathogenic potential not seen in isolates from other sources.

Emergence of Aspergillus sydowii as a marine pathogen implies that the land-sea boundary is not an effective barrier to disease transmission. Several Aspergillus species are known to cause opportunistic infections of animals; generally attacking individuals with weakened immune systems or other impaired defence systems. Sea fans may be infected opportunistically by Aspergillus sydowii, the fungus taking advantage of a host weakened by pollution or other environmental factors. Elevated temperature and nutrient pollution are among several factors that can contribute to coral diseases. Elevated temperature alters the host-pathogen interaction and is considered the key driver of coral disease outbreaks (Harvell et al., 1999; Kim & Rypien, 2015).

Aspergillosis in various animal species, and in humans too, has increased over the last few decades. Aspergillus species are found worldwide in humans and in almost all domestic animals and birds as well as in many wild species, causing a wide range of diseases from localised infections to fatal disseminated diseases, as well as allergic responses to inhaled conidia. Also, disease-causing isolates may possess a specific pathogenicity factor, perhaps a mycotoxin or other secondary metabolite, that is not present in all isolates. Apart from the invasive infections in sea fan corals described in this Section, other forms of animal aspergillosis that cause concern are: stonebrood mummification in honey bees, pulmonary and air sac infection in birds, mycotic abortion and mammary gland infections in cattle, guttural pouch mycoses in horses, sinonasal infections in dogs and cats, and invasive pulmonary and cerebral infections in marine mammals and nonhuman primates (Seyedmousavi et al., 2015).

Updated July, 2018